Rubber Grommet washer, looks like the same used on some cars. Autozone (or equivalent) may have something, especially if you can bring in the old washer. May even find them in the loose screw/nut drawers at Home Depot or Lowes

Iíve used a similar washer / insert to mount mirrors and exhaust stack brackets on trucks. I bought them from Lawson a hardware supplier. I still have a bunch somewhere the brass insert I think was for 1/4 x 20 threads.

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Ron on Northern Lights II
I don't like making plans for the day because the word "premeditated" gets thrown around in the courtroom.

My rule of thumb for hardware. If I can't find it at McMaster Carr, then engineer for something else. Cause it's just going to be worse next time. Besides, if McMaster does not have it, good chance it no longer exists. Just a rule of thumb.